1 We mapped the locations of live and dead trees in a large forest plot dom
inated by pioneer Douglas fir (Pseudotsuga menziesii) with an understorey o
f the invading late-successional species western hemlock (Tsuga heterophyll
a) and western red cedar (Thuja plicata) on Vancouver Island, British Colum
bia, Canada, to test for intra- and interspecific density-dependent effects
on tree survival.
2 We analysed both the spatial patterning of trees in the plot and the rela
tionships between neighbourhood density and tree survival. We also examined
the effects of additional variables (principally elevation) as covariates
in our neighbourhood analyses,
3 Both the spatial and initial neighbourhood analyses suggested strong intr
a- and interspecific density-dependent effects on tree survival, Douglas fi
r survival was significantly higher in less dense patches of conspecifics a
nd non-random tree death led to regularly spaced survivors, as expected fro
m intraspecific competition. The significantly lower survival of western he
mlock in denser patches of Douglas fir and the resulting negative spatial a
ssociation between surviving trees of these two species were consistent wit
h interspecific competition.
4 However, having controlled for the influence of elevation on tree surviva
l (probably mediated by variation in soil moisture) in neighbourhood analys
es, although the survival of the pioneer Douglas fir trees was still subjec
t to strong density-dependent effects., variation in its density in the ove
rstorey no longer appeared to influence the survival of the invading late-s
uccessional species, There was, however, evidence for asymmetric interspeci
fic density dependence between the two late-successional species since west
ern hemlock mortality tended to be higher in denser patches of western red
cedar.
5 Our results emphasize the importance of considering confounding factors i
n studies that seek evidence for density dependence.